摘要
频率合成器是电子系统的心脏，是决定电子系统性能的关键设备，在我们的生活中扮演着重要的角色。随着现代军事、国防及无线通信事业的发展，移动通信、雷达、制导武器、电子测量仪器和电子对抗等电子系统对频率合成器提出了越来越高的要求。世界各国都非常重视频率合成器的研究与应用，低相位噪声、高纯频谱、高速捷变和高输出频段的频率合成器已经成为频率合成发展的主要趋势。
本课题来源于成都赛英科技公司与国营第七八六厂合作项目：X、Ku波段宽带低噪声雷达跳频源。课题要求输出频段较高，相位噪声较低(-95dBc/Hz@1KHz)，且在宽带内实现跳频。根据频率合成理论，单环等简单的频率合成无法满足课题的要求。最终，课题采用了PLL+混频等综合频率合成技术。采用两个锁相环，一是数字环，实现主要的跳频功能；二是产生超低相位噪声的本振信号环，与数字环信号进行上混频实现所需频段，这样降低了数字环的倍频次数，减小相位噪声恶化，达到低噪声的效果。另外，数字环和模拟环同时跳频，覆盖整个宽带。
本文首先介绍了频率合成的发展历史及其最重要的指标：相位噪声，然后介绍了锁相环频率合成技术的基本理论以及在具体系统设计中应该注意的问题。接下来详细叙述了本课题所采用的方案，各模块功能的实现，环路滤波器的设计方法及实际设计中遇到的困难，最后给出了系统的最终测试结果及改进措施。
Frequency Synthesizer is the heart of all the electronic systems. It determines the performance of the electronic equipments, plays the most important role in our lives. Along with the development of wireless communication, modern military and national defence, many systems such as radars, homing weapons, measure equipments need higher and higher performance of frequency synthesizer, wide frequency range, low phase noise, high spurious restraining, low jumping time and high working frequency. Most countries in the world have been playing more attention on the design of it.
The subject comes from the cooperation project "X、Ku wide-band low noise radar hopping frequency synthesizer", between Chendu Sine Science and Technology Ltd. and No.786 national military factory. There are many difficulties in this project, such as high working frequencies, very low phase noise (-95dBc/Hz@1KHz ), wide band and hopping. Accordingly to the basic theory of frequency synthesis, we can't get the performance with just a singal PLL. Finally, the hybrid frequency sythesis technology: PLL+Mixer is adopted. We use two PLLs. One is a digital PLL which can achieve the mainly function of hopping. The other is to get a local oscillator with much lower phase noise.. Then we upconvent these two sigals in order to get high working frequencies we demanded. And both of these two PLLs have low multiples (N). So we get the low phase noise. Additionally, both of these two PLLs should hop synchronous to cover the wide band.
At first, the history and the important specifications (phase noise) of the frequency synthesis technology is reviewed. Subsequently, the theory of PLL frequency synthesis technology are introduced. Then the scheme of this subject, the realization of each functional module, and some questions which must be paid attention to in the system design are presented. Finally, the testing results is given and analyzed in detail.
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